My fingernails have taken a beating trying to remove what I could from it.
Art thought he'd try this Soft Scrub Total.
It has a spray and foam option. He thought the foam clung better.
We let it soak for a while and then scrubbed it with a teflon Dobie pad.
It didn't remove all of it, but most. I could also scrape off the left over deposits easier when it was dry.
I can live with this.